Rare-earth sintered magnets of Nd—Fe—B base primarily containing Nd as the rare-earth element R have high magnetic characteristics and have been used in VCMs (voice coil motors), MRIs (magnetic resonance imaging apparatuses), and other various fields. These magnets have sizes with one side of several millimeters to several tens of millimeters. Cylindrical magnets having outer diameters of 3 mm or less are used in vibration motors for cellular phones, and further minute magnets are required in the fields of micromachines and sensors. For example, a flat-shaped magnet having a thickness of 1 mm or less is prepared through the steps of cutting from a somewhat large sintered block, polishing, or the like in advance. However, it is difficult to produce a magnet of 0.5 mm or less because of a magnetic strength problem or a productivity problem.
On the other hand, recently, thin film magnets in minute sizes have become prepared by physical film formation methods, e.g., sputtering and laser deposition, and for the magnetic characteristics, a maximum energy product of 200 kJ/m or more has been reported (for example, Non-Patent Document 1 and Patent Document 1). According to these preparation methods, magnet alloy components are deposited on substrates or shafts in a vacuum or in a space at a reduced pressure, and are subjected to a heat treatment, so that a high performance film exhibiting about 200 kJ/m can be produced by a simple process relative to a sintering method by appropriately controlling various conditions.
As a general example, the thickness of the thin film magnet formed on a base material, e.g., a flat plate or a shaft, is about several micrometers to several tens of micrometers, and in many cases, it is one-several tenth to one-hundredth of the four sides of the flat plate or the diameter of the shaft. When this thin film is magnetized in a direction perpendicular to the flat plate surface or the circumferential surface of the shaft, a demagnetizing field is increased significantly, and adequate magnetization is not performed. Therefore, it becomes difficult to exploit the magnetic characteristics inherent in the thin film magnet. It has been generally known that the magnitude of the demagnetizing field depends on the ratio of the dimension of magnet in the magnetization direction to the dimension in the direction orthogonal thereto, and is increased as the dimension in the magnetization direction (=film thickness direction) is decreased.
On the other hand, if an easy-to-magnetize magnet material can be prepared from a point of view different from the above-described dimensional ratio problem, it becomes possible to exploit the characteristics of the thin film magnet easily. Consequently, the usefulness is exerted in preparation of various application devices. In a method generally adopted for known Nd—Fe—B based thin film magnet, components constituting the magnet are deposited in an atomized or ionized state on a base material, and Nd2Fe14B crystal grains of less than 0.3 μm corresponding to a single-magnetic-domain grain diameter are generated by the following heat treatment (Patent Documents 2 and 3).
At this time, in general, it is a common means to control the crystal grains at small size so as to obtain desired magnetic characteristics (for example, Patent Document 4). However, there is almost no document in which the crystal grain diameter and the magnetization characteristics are discussed. If the crystal grains are grown to 0.3 μm or more, the inside of each crystal grain takes on a multidomain structure and, thereby, the coercive force is reduced.
For the purposes of reference to evaluation of the magnetization characteristics, FIG. 1(a) shows an initial magnetization curve and a demagnetization curve of a general sintered magnet, and FIG. 1(b) shows an initial magnetization curve and a demagnetization curve of a known example of thin film magnet. As is clear from FIG. 1(a), when a magnetic field is applied to the sintered magnet, the magnetization rises steeply, and adequately high magnetization characteristics are exhibited even when the magnetic field is at a low level of about 0.4 MA/m.
On the other hand, for the thin film magnet of a known example shown in FIG. 1(b), the magnetization is increased gradually from an origin point, and no tendency of saturation is observed even at a magnetic field of 1.2 MA/m. The reason for this difference in magnetization characteristics is estimated that the sintered magnet has a nucleation type coercive force mechanism whereas the thin film magnet of the known example is based on the single-magnetic-domain grain type coercive force generation mechanism.
Non-Patent Document 1: Journal of Magnetics Society of Japan, Vol. 27, No. 10, 1007, (2003)
Patent Document 1: Japanese Unexamined Patent Application Publication No. 8-83713
Patent Document 2: Japanese Unexamined Patent Application Publication No. 11-288812
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2001-217124
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2001-274016